[dm-devel] [PATCH v2] dm: reduce the number of processes per dm device

[Mikulas Patocka]

This is updated patch to reduce the number of processes. When we free 
q->bio_split bioset, we must also avoid the call to blk_queue_split that 
uses it. The call to blk_queue_split is useless because device mapper does 
its own splitting.


From: Mikulas Patocka <mpatocka at redhat.com>

The patch 54efd50bfd873e2dbf784e0b21a8027ba4299a3e ("block: make
generic_make_request handle arbitrarily sized bios") makes it possible for
block devices to process large bios. The patch allocates a new bio set
queue->bio_split for each device, this bio set is used for allocating bios
when the driver needs to split large bios.

Each bio_set allocates a workqueue process, thus the above patch increases
the number of processes allocated per block device.

Device mapper doesn't need the queue->bio_split bio_set, thus we can
deallocate it. This reduces the number of allocated processes per
dm-device from 3 to 2. We remove the call to blk_queue_split, it is not 
needed because device mapper does its own splitting.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/md/dm.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Index: linux-4.3-rc6/drivers/md/dm.c
===================================================================
--- linux-4.3-rc6.orig/drivers/md/dm.c	2015-10-21 19:29:04.000000000 +0200
+++ linux-4.3-rc6/drivers/md/dm.c	2015-10-21 19:29:58.000000000 +0200
@@ -1742,8 +1742,6 @@ static void dm_make_request(struct reque
 
 	map = dm_get_live_table(md, &srcu_idx);
 
-	blk_queue_split(q, &bio, q->bio_split);
-
 	generic_start_io_acct(rw, bio_sectors(bio), &dm_disk(md)->part0);
 
 	/* if we're suspended, we have to queue this io for later */
@@ -2770,6 +2768,12 @@ int dm_setup_md_queue(struct mapped_devi
 	case DM_TYPE_BIO_BASED:
 		dm_init_old_md_queue(md);
 		blk_queue_make_request(md->queue, dm_make_request);
+		/*
+		 * We don't need the bioset, so we free it to save one process
+		 * per device.
+		 */
+		bioset_free(md->queue->bio_split);
+		md->queue->bio_split = NULL;
 		break;
 	}